1. Field of the Invention
The invention relates generally to GPS receivers and more particularly to a GPS receiver having a fast time to first fix by comparing GPS satellite range rates measured from the GPS signal carriers to GPS satellite velocities calculated from coarse GPS satellite orbital parameters even when the orbital parameters are not current.
2. Description of the Prior Art
The global positioning system (GPS) is a satellite based location and time transfer system developed by the United States government and available free of charge to all users. A GPS user location is based upon one-way ranging between the user and GPS satellites. The GPS satellites transmit signals having the times-of-transmission and orbital parameters for their respective time variable locations-in-space. A GPS receiver acquires the GPS signals by correlating internal replica signals to the carrier frequencies and distinguishable codes for each of several in-view GPS satellites. When the GPS signals have been acquired, the GPS receiver uses the times and orbital parameters for measuring ranges to four or more GPS satellites. These ranges are called pseudoranges because they include a term caused by a time error of the internal clock in the GPS receiver.
The GPS satellite pseudoranges are measured by determining phase offsets between GPS pseudorandom (PRN) codes received in the GPS signals and internal GPS replica PRN codes referenced to the internal clock. Some GPS receivers measure and integrate the carrier phases of the GPS signals in order to reduce the noise on the measured phase offsets. The GPS receiver then determines a GPS-based time by monitoring the GPS signals until a Z-count is decoded. The GPS-based time is used to determine the times that the phase offsets were measured. The measurement times are then used with ephemeris data that is received in the GPS signals for calculating the instantaneous locations-in-space of several GPS satellites and for linearizing location equations relating the calculated locations-in-space to the measured pseudoranges. Having four or more linearized location equations for four or more GPS satellites, respectively, the GPS receiver can resolve the three dimensions of geographical location of the GPS receiver and correct the error in the internal clock time.
Two types of orbital parameters are transmitted for determining locations-in-space for the satellites: almanac data and ephemeris data. The almanac data includes relatively few parameters and is generally sufficient for determining locations-in-space to a few kilometers. Each GPS satellite broadcasts the almanac data for all the GPS satellites on a twelve and one-half minute cycle and is updated every few days. Almanac data is typically used by GPS receivers that have been off for more than a few hours for determining which GPS satellites are in-view and is sometimes used for determining lines-of-sight to the satellites for the linearized location equations. However, the GPS almanac data is generally not used for determining the pseudoranges in the linearized location equations because the inaccuracy of the location-in-space of a GPS satellite transfers to an inaccuracy of the user location for the GPS receiver. In the past, some GPS receivers have used almanac data for pseudoranges for determining a user location. However, the user location accuracy using almanac-based pseudoranges was not satisfactory, and so far as is known this technique is no longer in use.
The ephemeris data provides relatively more parameters and is much more accurate. Typically, current ephemeris data is sufficient for determining locations-in-space to a few meters without selective availability or a few tens of meters at current levels of selective availability. Each GPS satellite broadcasts its own ephemeris data on a thirty second cycle. Ephemeris data is updated each hour. However, after about two hours the accuracy of the ephemeris data begins degrading. Typically, ephemeris data that is more than about four hours old is avoided for determining the pseudoranges for a user location.
A fast time to first fix, defined as the first accurate location fix after a several hours without location fixes, is desirable or required in certain applications for GPS receivers. For example, it is desirable for a vehicle navigation GPS receiver that has been overnight in a parking garage and then driven onto a street to show a location as soon as possible for customer satisfaction. For vehicle tracking for fire engines, ambulances, other dispatched vehicles, and the like, it is essential for the tracking station to beginning tracking as soon as possible and highly desirably to at least know if the vehicle has turned right or left upon leaving a station or garage. GPS receivers in vehicle navigation and vehicle tracking are typically coupled with map matching and inertial navigation in order to fill in gaps when driving due to the GPS signal being blocked. Simply knowing whether the vehicle has turned right or left after it leaves a garage or station is a great aid to the map matching algorithms. In certain vehicle tracking applications, such as fire and ambulance service, the tracking dispatcher can save valuable seconds just by knowing if a service vehicle has turned the wrong way out of a station.
Inertial navigation devices such as gyro-compasses can become disoriented during overnight inactivity or in the multiple circles of a multi-level parking garage. Upon leaving the garage a fast time for determining a new heading direction is desirable in order to re-initialize the gyro-compass or inertial device to begin accurate operation as soon as possible.
In a typical GPS receiver, the time to acquire new ephemeris data is a major portion of the time to a first location fix and to a determination of a first new heading direction. Typically, the ephemeris data is obtained directly from the GPS satellites in the GPS signals. However, up to about thirty seconds is required to acquire ephemeris data in this manner. Several proposals to eliminate this thirty seconds have been made. First, the GPS signals can be rebroadcast to a vehicle inside of the garage or station so that the GPS receiver can continue to get updated ephemeris. However, such re-broadcasting requires a costly GPS repeater. Second, if location is not needed in the vehicle, the location of the vehicle can be computed at a base station where current ephemeris data is available. In this case, raw pseudoranges are sent to the base station along with the satellite identifications and times. Third, if the location is needed at the vehicle, the base station sends the current ephemeris data including the satellite identifications and times to the vehicle for location determination. The latter two schemes can be attractive for systems where radio equipment is already required. Unfortunately, many applications for GPS receivers, including vehicle navigation, do not otherwise require a radio. Even in applications where a radio is required, such as vehicle tracking, the tracking system may not be set up for transmitting ephemeris data or receiving raw pseudoranges. Further, a relatively long string of data is required for radio transmission of ephemeris. In summary, all these proposals for providing new ephemeris data in some other way than receiving it in the GPS signal have limitations.
There is a need for an GPS receiver having a fast time to first fix without the use of new GPS ephemeris data.